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If ActivityPub is “simple” then that’s a recent’ish development. It used to be a mess.

I would imagine AT gets easier to use/setup given time too.

They said comparatively simple. As in, simple compared to AT

You can do a basic[0] AP server in a single small (64k) PHP file[1].

[0] Single user, admittedly, but if we're talking about protocol simplicity, that's the nub.